1. Field of the Invention
The present invention relates to a skew correction device, an image forming system, and a skew correction method.
2. Description of the Related Art
There are widely known sheet post-processing apparatuses each of which is disposed on the downstream side of an image forming apparatus main unit and performs post-processing such as binding on recording sheets or the like output from the image forming apparatus. Nowadays, such sheet post-processing apparatuses have been multifunctionalized, and generally perform a bookbinding process in which not only conventional edge binding but also saddle-stitching is performed. In some of the saddle-stitching bookbinding processes, a cutter cuts an edge of a bound booklet in order to further improve output quality.
In the cutting process of such a cutter, the booklet is conveyed by a conveying unit, such as a belt, and subjected to a skew correction by being pressed from the trailing end side thereof in the sheet conveying direction to be placed in contact with an abutting stopper placed in accordance with factors such as a size and an cutting amount of the booklet. For example, Japanese Patent No. 4134854 discloses such a skew correction technique.
In Japanese Patent No. 4134854, a leading end in the conveying direction of a booklet carried onto a booklet placing table is received by a pressing member of a booklet conveying unit. The pressing member is moved to place a trailing end in the conveying direction of the booklet in contact with a reference alignment member that can be turned up and down, and thus, a skew in the conveying direction of the booklet is corrected. The booklet is pressed by a pressing unit while being in contact with the reference alignment member, and thus is inserted into a cutter while maintaining the attitude when being in contact with the reference alignment member, and a cutting process is performed.
According to Japanese Patent No. 4134854, as described above, the skew in the conveying direction of the booklet is corrected by placing the trailing end in the conveying direction of the booklet in contact with the reference alignment member that can be turned up and down. However, although the booklet is not smeared or wrinkled due to friction between the belt and the booklet when the booklet is carried by the belt or the like and corrected in the skew thereof from above and below, it is not always possible to successfully place the booklet in contact with the reference alignment member due to deflection or buckling toward upside of the booklet occurring when the pressing member presses the booklet, because there is no control member at the upside of the booklet.
More specifically, when skew correction and positioning of the booklet is performed, the booklet is placed in contact with a positioning stopper by a trailing end jogger plate (pressing plate) or the like and subjected to the skew correction. When the trailing end jogger pushes the trailing end (end face side) of the booklet, a deflection may occur resulting in a buckling in an uncontrolled direction of the booklet depending on the sheet type and stiffness of the booklet, and thus, there is a case in which the booklet is difficult to be pressed in the horizontal direction. In such a case, the skew correction of the booklet is not achieved because a predetermined amount against the abutting stopper is not obtained. In the invention disclosed in Japanese Patent No. 4134854, the uncontrolled direction corresponds to the upside of the booklet.
This will be described specifically. FIGS. 20 and 21 illustrate the states of a conventional general skew correction. In FIG. 20, a guide plate 318 for a booklet BT is located only below the booklet BT, and the booklet BT moves along the upper surface of the guide plate 318. FIG. 20 illustrates an operation when the booklet BT is subjected to the skew correction by being pushed by a jogger 319 to the side of a positioning stopper 317 located on the downstream side of the guide plate 318; (a) illustrates a state in which the booklet BT is in an initial position; (b) illustrates a state in which the booklet BT is moving; and (c) illustrates a state in which the leading end of the booklet BT abuts against the positioning stopper 317, and subjected to the skew correction. Note that, in this specification, the term “abut” means bumping against an object and maintaining the abutting (contacting) state.
The jogger 319 pushes the end face side (trailing-end side) of the booklet BT in the direction indicated by arrow D1. The pushing amount is an amount L1 determined by factors such as the size, the number of sheets, and the type of sheet of the booklet BT. The amount L1 is longer than a theoretical distance L2 from the leading end of the booklet to the positioning stopper 317 by a predetermined amount (L1>L2+α). By this setting, the booklet BT can be pressed by the predetermined amount after the leading end of the booklet BT has abutted against the positioning stopper 317, whereby the skew is corrected even if the booklet BT is obliquely traveling.
FIG. 21 illustrates an operation in the case in which the skew correction of the booklet BT is not successfully performed. Illustrated in (a) of FIG. 21 is a state in which the booklet BT is in the initial position; illustrated in (b) of FIG. 21 is a state in which the booklet BT is moving; and illustrated in (c) of FIG. 21 is a state in which the leading end of the booklet BT is supposed to abut against the positioning stopper 317, positioned, and skew corrected, but is actually not.
The surface of the jogger 319 for pushing the end face side of the booklet BT has generally a flat-surface shape. However, some types of sheets lack stiffness, and in that case, as illustrated in FIG. 20B, a force in the horizontal direction (direction of arrow D1) acts above the booklet BT because of the lack of stiffness. Consequently, a deflection is produced by this horizontal force, and the booklet BT buckles as illustrated in FIG. 21C. As a result, the horizontal moving amount of the booklet BT becomes smaller, whereby the positioning cannot be performed correctly. When the positioning is not performed correctly, the skew correction (alignment) of the booklet cannot be performed either.
Therefore, there is a need for a skew correction to be performed without particularly adding a controlling unit, such as a control plate, when correcting a skew of a booklet by pushing the trailing end of the booklet.